The present invention relates to catalyst compositions for purifying terephthalic acid, based on Group VIII metals, to methods for preparing thereof, and to a method of purifying terephthalic acid suitable for the subsequent synthesis of polyester polymers and copolymers used in the manufacture of textile fibers.
It is of importance that terephthalic acid which is used as a monomer in the manufacture of polymer fibers should have a high purity. The main controllable quality parameters of pure terephthalic acid are the content of p-carboxybenzaldehyde and colored impurities in it.
Purified terephthalic acid is derived from less pure, technical grade or xe2x80x9ccrudexe2x80x9d terephthalic acid by hydrofining the latter (treating in the presence of hydrogen) over Group VIII metal catalysts. Crude terephthalic acid is dissolved in water at an elevated temperature, and the resulting solution is hydrogenated in a vibrated reactor or in a reactor with a stationary bed, preferably in the presence of Group VIII metal catalysts. The methods of purification, composition of the catalysts, and processes for preparing these catalysts are described in numerous patents.
The activity and selectivity of catalysts for the hydrofining of terephthalic acid depend on a great number of factors, such as the content of Group VIII metal(s) in the catalyst, the type of support, the method of applying Group VIII metal(s) to the support, and also on the distribution of the metal or metals of Group VIII were applied to a support, and also on the distribution of said metal(s) over the granule of the carrier.
Known in the art is a method of hydrofining terephthalic acid [UK Patent No. 994769, 1965], wherein a palladium-on-active carbon catalyst composition displays high activity in the reaction of purifying terephthalic acid from p-carboxy-benzaldehyde impurities. As supports for palladium other compounds, such as SiO2, Al2O3, were also investigated. It was found that carbon carriers were the best, because they, in contrast to oxide carriers, are not subject to rapid degradation in corrosive hot aqueous solutions of terephthalic acid.
The results of investigating the influence of the nature of carbon carriers are known. It has been shown that active carbons prepared from vegetable, animal or mineral sources, preferably from coconut active carbon, are suitable for the production of palladium catalysts for the hydrofining of terephthalic acid. It is desirable that the surface area of such active carbons should be at least 600 m2/g, and the size of granules should correspond to 3-6 mm. In U.S. Pat. No. 4,728,630, 1988 an additional characteristic of such active carbon is introduced, namely, the pH value of its aqueous suspension. In SU Patent No. 1660282, 1997 a possibility is disclosed of using, as a suitable carrier, an active porous carbonaceous material modified with pyrocarbon. In U.S. Pat. Nos. 4,415,479, 1983; 4,421,676, 1983; and 4,791,226, 1988 it is indicated that for a more effective process of the hydrofining of terephthalic acid from p-carboxybenzaldehyde it is important to prepare catalysts with a definite size of the particles of applied palladium. The size of such particles must be not greater than 35 xc3x85. The authors of U.S. Pat. Nos. 4,394,299, 1983 and 4,791,226, 1988 also point out a positive effect of such distribution of palladium particles in the granule of a carbon material, when they predominate on the outer surface of the granule.
In many patents it is pointed out that along with monometallic catalysts, the incorporation of Ni, Co, Cu, Fe, Mn, U, Cr, as well as Ir, Rh, Pt and Ru, into the catalyst composition may produce positive effect on the catalytic activity of palladium.
According to other group of patents [U.S. Pat. Nos. 4,629,715, 1986 and 4,892,972, 1990], the most effective action of bimetallic catalysts is attained, when the catalysts are arranged in a reactor in layers, for instance, Pd/C and Rh/C instead of one layer (Rh+Pd)/C. The authors of U.S. Pat. No. 4,892,972, 1990 even claim a method with the use of a layered catalyst bed, e.g., Ru/C+Rh/C+Pd/C.
Usually, catalysts comprising Group VIII metals, particul-arly palladium catalysts, are prepared by the adsorption of a palladium salt from solution to the carrier. In one of the processes [U.S. Pat. No. 2,857,337, 1967] such salt is treated with a water-soluble metal hydroxide or with a basic carbonate, this being followed by the reduction to metallic palladium with the help of such reducing agents as formaldehyde, glucose, glycerol, etc.
According to Keith et al. [U.S. Pat. No. 3,138,560, 1964], on addition of sodium tetrachloropalladoate or palladium chloride to many of carbon carriers, a large part of palladium immediately precipitates in the form of lustrous film of metallic palladium. Catalysts prepared by such a method usually have low activity. An opinion was voiced that palladium directly reduces to metal owing to free electrons or to the presence on the carbon surface of such functional groups as aldehydes. Palladium catalysts in the step preceding the reduction are predominantly prepared by fixing palladium in the form of an insoluble compound, so to avoid the problems of migration of palladium particles and growth of crystallites which may originate during the reduction of palladium from solution.
Though p-carboxybenzaldehyde is the most harmful impurity, which is crucial for the quality of the terephthalic acid used for the manufacture of plastics and sharply deteriorates the quality of the latter, p-toluic acid (p-TA) is also an undesirable impurity, which must be removed from the aqueous solution of terephthalic acid, obtained as a result of hydrofining. Though such removal can be achieved to a large extent owing to the greater solubility of p-toluic acid as compared to terephthalic acid, in water nevertheless substantial amounts of p-toluic acid are trapped within purified terephthalic acid crystals in the step of terephthalic acid crystallization from solution.
To avoid this disadvantage attendant to the separation of p-toluic acid, it has been proposed to decarbonylate p-carboxybenzaldehyde in aqueous solutions to benzoic acid in the presence of a palladium-on-carbon catalyst, since benzoic acid is more soluble in water than p-toluic acid [U.S. Pat. No. 3,456,001, 19691]. However, the foregoing decarbonylation of p-carboxybenzaldehyde to benzoic acid produces equimolar amounts of carbon monoxide, a well-known poison for the noble metals such as palladium [U.S. Pat. No. 4,201,872, 1980]. To minimize catalyst poisoning, in the aforementioned US Patent it is proposed to carry out the decarbonylation at relatively low process pressures so as to minimize dissolved carbon monoxide concentration in the liquid reaction medium. The process pressure also must be controlled within a closely defined pressure range. The generated carbon monoxide is purged from the reactor as a gas.
It is known [U.S. Pat. No. 4,892,972, 1990], that the use in the aforesaid purification of crude terephthalic acid of a catalyst system comprising a first layer of catalyst particles containing a metal of Group VIII of the Periodic Table of Elements supported on a carbon carrier and a second layer of palladium-on-carbon catalyst particles and the passage of the aqueous solution of crude terephthalic acid through the aforesaid first layer of rhodium-on-carbon catalyst particles and then through the second layer of palladium-on-carbon catalyst particles permits the amount of p-toluic acid produced during purification of crude terephthalic acid to be minimized. Such method of using the aforesaid catalyst system does not promote the hydrogenation of p-carboxybenzaldehyde to p-toluic acid but instead promotes the decarbonylation of p-carboxybenz-aldehyde to benzoic acid, which is more soluble than p-toluic acid in water and thus is more readily separable than p-toluic acid from terephthalic acid upon crystallization of the latter. This permits a feed solution having a relatively higher p-car-boxybenzaldehyde content to be processed more economically.
The closest method of purification is described in GB Patent No. 1578725, 1980, wherein the authors propose to use catalysts comprising 2 or more metals such as Pt, Pd, Rh, Ru, Os, Ir, Fe, Ni, Co, Cr, Mn and U, in which one of the metals is Pd or Pt. In said catalysts metals are in the form of an alloy, a physical mixture, or are applied to an active carbon support (3 to 6 mm granules). Hydrofining is carried out by treating a terephthalic acid solution with hydrogen in the presence of said catalysts at elevated temperatures (280xc2x0 C.) and a pressure (xcx9c100 atm.). The rate of hydrogenation in the presence of a bimetallic catalyst (0.4% Pd-0.1% Pt)/C per gram of the catalyst is 20% higher than with the use of 0.5% Pd/C.
So, crude terephthalic acid containing p-carboxybenz-aldehyde and other impurities can be purified by hydrogenation over conventionally prepared catalysts based on Group VIII metals applied to carbon.
The present invention solves the problem of providing selective and stable catalysts and processes, wherein crude terephthalic acid with a high initial content of p-carboxybenz-aldehyde would be selectively hydrogenated and/or decarboxylated into benzoic acid with a low residual content of p-carboxybenz-aldehyde.
Said problem is solved in the following manner: by using in a method for the purification of terephthalic acid a catalyst composition comprising crystallites of catalytically active palladium or palladium and at least one metal of Group VIII of the Periodic Table of Elements, applied to the surface of a carbon material, wherein a mesoporous graphite-like material with the average mesopore size in the range of from 40 to 400 xc3x85, the proportion of the mesopores in the total pore volume of at least 0.5, and the degree of graphite-similarity of at least 20% is used as the carbon material, in which metal crystallites are distributed in the volume of the carbon material granules in such a manner that the distribution peaks of these crystallites should be at a distance from the outer surface of the granule corresponding to 1-30% of its radius.
The catalyst composition comprises crystallites of palladium and rhodium or of palladium and ruthenium, or of palladium and platinum; the total content of the metals varies within the range of from 0.3 to 3.0 percent by weight, the weight ratio of palladium to other metals varies within the range of from 0.1 to 10.0
The problem is also solved by the provision of a method of preparing a catalyst composition for the purification of terephthalic acid by applying catalytically active palladium or palladium and at least one of Group VIII metals to the surface of granules of a carbon carrier, with said granules being in contact with an aqueous solution of palladium salts or palladium salts and at least one of Group VIII metals to produce a metal salt-porous carbon precursor, in which method the precursor is dried and treated with a reducing agent in an amount sufficient for reducing the superficial metal salts to metal crystallites, wherein a mesoporous graphite-like material with the average mesopore size in the range of from 40 to 400 xc3x85, the proportion of the mesopores in the total pore volume of at least 0.5, and the degree of graphite-similarity of at least 20% is used as the carbon material to produce a monometallic or bimetallic catalyst.
Said catalyst composition is prepared with the use of one of the following metal precursors:
H2PdCl4 or Pd(NO3)2;
H2PdCl4 or RuOCl3 or RuNO(NO3)3;
Pd(NO3)2 and RuOHCl3 or RuNO(NO3)3.
For preparing said catalyst composition, nitrate solutions of palladium and/or ruthenium salts are prepared with concentration of free nitric acid from 37 to 170 g/l. Bimetallic catalysts are prepared by applying metal precursors simultaneously or in succession.
We have found that such catalyst can be prepared, if mono- or bimetallic particles of Group VIII metals are applied to the surface of carbon materials having an average pore size of 40 to 200 xc3x85 and a considerable (from 20 to 60%) degree of graphite similarity; said metallic particles being distributed within the volume of the carbon carrier in such a manner that their distribution peaks are at a distance from the outer surface of said granule, equivalent to 1-30% of its radius.
As the aforesaid carbon materials carriers may be used, prepared by the heat treatment of plastics, and also synthesized in accordance with a special technology from gaseous hydro-carbons (V. A. Likhoborov et al., React. Kin. Cat. Lett., vol. 54, 2 (1995) 381-411), namely, Sibunit, KVU and various compos-ites based thereon. The physicochemical characteristics of some carbons are presented in Table 1. The tabulated data indicate that the aforesaid carbon materials in the set of such parameters as Vmeso/Vxcexa3 and K sharply differ from conventional active carbons usually employed for producing catalysts for the hydrofining of terephthalic acid, prepared from vegetable, animal or mineral sources, preferably coconut active carbons tat are used for preparing conventional terephthalic acid hydrofining catalysts.
We have also found that if in the pores of such carbon materials the distribution of metal particles over the volume of the carrier granule is effected in such a manner that the distribution peak(s) will be found at as distance from the outer surface of the granule corresponding to 1-30% of its radius, then such catalyst has an enhanced service life on reuse. This effect is particularly manifest, when palladium and ruthenium are used as Group VIII metals. Furthermore, the presence of both palladium and ruthenium in the catalyst leads to the effect of synergism, rather than additive growth of the catalyst activity, especially if the distribution peaks of the particles of these metals are in the area adjacent to the outer surface of the catalyst granules.
We have also found that with such distribution of palladium and ruthenium particles in the granule of the mesoporous carbon material it is possible to replace a part of palladium by ruthenium, this leading not only to lower costs of the catalyst (since ruthenium is substantially cheaper than palladium), but also to a change in the ratio of concentrations of p-toluic and benzoic acids, which are the products of p-carboxybenzaldehyde conversion, towards the latter, this promoting the attaining of a higher quality of the obtained crystalline terephthalic acid.
For preparing the aforementioned catalysts, i.e., the cata-lysts containing mono- or bimetallic particles of palladium and ruthenium, applied to the surface of carbon carriers, it is possible to use such methods well-known in the literature as impregnating the carrier with solutions of various salts of palladium and ruthenium. However, as has been found, the best catalysts are obtained by using a method of spray-depositing acid solution of palladium and ruthenium salts on a suitable carbon carrier with subsequent treatment of the thus applied metal precursors with hydrogen.